The present invention relates to thermostats for controlling the level of operation of one or more systems to correspond with a time-of-use energy rate, and to thermostats that can provide demand side management control to an electric utility provider.
As the demand for electrical power increases during the day, the utility provider experiences an increase in the cost of generating electrical power as a result of secondary “peak” power plants that are switched on to supplement off-peak power generating plants. Many utility providers are consequently establishing real time energy price rates for charging the consumer a variable rate that increases as the cost of generating power increases during peak demand periods. These rates may vary based on a utility price rate schedule, which establishes a different usage price rate for a plurality of specified time periods of the day. Such schedules are also periodically changed by the utility throughout the year. In situations where the peak demand begins to exceed the power generating capacity of the utility's off-peak and peak power plants, the utility may engage in demand side management, which curtails or reduces consumer energy usage to keep the demand from exceeding capacity. Utilities engaging in demand side management transmit a signal to various users of electrical energy to reduce the amount of energy they use during peak demand periods by turning off electrical loads such as air conditioners. In the example of an air conditioner controlled by a conventional thermostat, the user would be billed at a higher rate when the air conditioner runs during peak energy demand periods. Previous attempts have been made to provide a curtailment thermostat that can offset the temperature set point to reduce the amount of energy used during peak demand periods. This type of curtailment thermostat attempts to reduce a consumer's energy usage when energy costs are high. Another type of curtailment thermostat receives a signal from a utility requesting a substantial offset to the temperature setting during a critical peak demand period, which will cause the air conditioner to immediately shut off and remain off until the temperature in the space rose above the substantially offset temperature setting. This would allow the utility to be able to lower energy consumption to keep the peak demand from exceeding their capacity, and the user would be able to save on their energy bill. However, this method of offsetting the temperature setting of a thermostat by a predetermined amount cannot always be relied upon to reduce the operating level of the air conditioner to reduce energy consumption. A consumer may lower the desired temperature setting for the space in anticipation of an offset override by the utility. For example, if the occupant of a space at 75 degrees Fahrenheit lowers the desired temperature setting to 70 degrees in anticipation of a utility offset request, the utility's 6 degree offset request would then raise the temperature setting to 76 degrees. If the temperature in the space subsequently increased to 76 degrees, the air conditioner would be activated to cool the space. If the particular air conditioner does not have the capacity to cool the space much lower than 76 degrees when outside temperatures are in excess of 95 degrees Fahrenheit, the air conditioner may operate for a long period of time. In this case, the utility would not have succeeded in curbing energy consumption during a peak demand period.